Kupffer cells are macrophages that line the hepatic sinusoids. These cells function to remove foreign and particulate matter from the portal circulation. In response to tissue injury, Kupffer cells become activated. They display enhanced biochemical and functional reactivity and release highly reactive and toxic mediators. We have been studying the potential role of activated Kupffer cells and their mediators in chemically induced hepatotoxicity. For these studies, we focused on the analgesic, acetaminophen as a model hepatotoxicant. Twenty-four hr following treatment of rats with acetaminophen, we observed an infiltration of mononuclear cells into centrilobular regions of the liver in the absence of necrosis. We hypothesized that these infiltrated cells consisted of newly recruited and "activated" macrophages and that they contribute to acetaminophen hepatotoxicity. In support of this hypothesis, we demonstrated that Kupffer cells from the livers of acetaminophen treated rats display morphological and functional characteristics of activated macrophages. We also showed that macrophage accumulation and activation in the liver is mediated, in part, by a factor (H-MAF) released from acetaminophen injured hepatocytes. In addition, Kupffer cells that accumulate in the liver following acetaminophen treatment are nonspecifically activated to kill hepatocytes. Taken together, these data support our model that activated liver macrophages contribute to hepatotoxicity. It is the purpose of the present proposal to extend these findings. We plan to analyze biochemical mechanisms of Kupffer cell activation following hepatotoxicant treatment. We will also use the fluorescence activated cell sorter to characterize soluble immune mediators released by Kupffer cells and hepatocytes, and to examine the interaction of Kupffer cells with other inflammatory cells in the liver. These studies should provide clues on the mechanisms underlying the contribution of Kupffer cells to chemically-induced hepatotoxicity.